1. Field of the Invention
The present invention relates to a device for the purification of exhaust gases produced by a multiple cylinder engine and, more particularly, to an exhaust gas purification system for use with a multiple cylinder engine equipped with two independent exhaust systems, each exhaust system being provided with an exhaust sensor.
2. Description of Related Art
Typically, in multiple cylinder engines, such as a V-type engine which has an exhaust system connected to each of two groups of cylinders and an exhaust gas sensor disposed in each exhaust system for detecting the concentration of oxygen within exhaust gases, an engine control system detects an air/fuel ratio of a fuel mixture supplied into each group of cylinders based on the concentration of oxygen within exhaust gases in each exhaust system. Such an engine control system also typically controls a fuel system so that the fuel mixture attains a target air/fuel ratio. Such an engine control system is known from, for example, Japanese patent application No. 62-162,727, entitled "Air/Fuel Ratio Control System," filed on Jun. 30, 1987 and now published as Japanese Unexamined Patent Publication No. 64-8,332.
It is also known from, for instance, Japanese Unexamined Patent Publication No. 60-231,155 to dispose an exhaust sensor downstream of a catalytic converter for purifying exhaust gases in an engine exhaust system. This sensor is used by the system to determine the state of deterioration of the catalyst from a signal produced by the sensor which is representative of exhaust gases.
Such exhaust gas sensors as those used to control the air/fuel ratio of a fuel mixture in the manner described have a detection performance which deteriorates with the passage of time. This results in a "dull" reaction of the exhaust gas sensor to changes in the air/fuel ratio, which can cause a deviation of a controlled air/fuel ratio from a target air/fuel ratio, thereby reducing exhaust gas purification performance.
If the exhaust gas sensor or sensors deteriorate, in an ordinary air/fuel ratio feedback control, what is known as an "inversion cycle" tends to become longer. Because of this, based on the fact that the inversion cycle has become longer than a specified inversion cycle under specific conditions, deterioration of the exhaust gas sensor can be judged to have occurred. However, it is possible that the exhaust gas sensor will be mistakenly judged to have deteriorated, based on its signal, if the change cycle of the air/fuel ratio itself has lengthened due to various other control factors. Therefore, if an inversion cycle, used as a deterioration determination standard, has a large value, then the accuracy of determining deterioration of the exhaust gas sensor is lowered. As a result, the exhaust gas purification continues to be poor. On the other hand, if the inversion cycle, used as a deterioration determination standard, is shortened, erroneous determinations that the exhaust sensor has deteriorated may occur.
When two independent exhaust systems are provided for an engine having two groups of cylinders, such as a V-type engine, an exhaust sensor is provided in each independent exhaust system in order to control the air/fuel ratio of a fuel mixture. In such an engine, it is desired, from a cost and service standpoint, to ascertain a state of deterioration of each exhaust gas sensor in a fairly simple way.